The present invention is directed to a method and apparatus for monitoring the position of an optical switch and detecting errors in the transmission quality of the signal being transmitted through it. The present invention is also directed to a plurality of such apparatuses being used to monitor a plurality of switches within an optical cross connect.
The demand for both greater volume and speed on long distance telecommunications networks has resulted in the rapid improvement of point to point optical transport systems (OTS). These systems can now transport data at rates greater than 20 Gb/s along a single fiber. As a result, demand for systems to provision this traffic and restore it in the event of a network failure has increased as well. The issue of restoration is further complicated by the need for it to occur very rapidly, on the time scale of a few seconds or less, and thus detection of the network faults that make restoration necessary must also be performed very rapidly.
One potential solution to this detection problem is through the utilization of xe2x80x9copaquexe2x80x9d networks with optoelectronic transponder interfaces to perform fault detection as illustrated in FIG. 1. In this configuration, a signal is received along optical fiber 101 and fed into transponder 102. Transponder 102 is an optoelectronic device that translates the optical signal into an electronic signal, performs tests for loss of signal, loss of frame, etc., then translates the signal back into an optical signal, and sends it on to optical cross connect (OXC) 103. If transponder 102 detects an error, it notifies OTS management system 104 which in turn notifies network management system 105. Network management system 105 notifies OXC management system 106 which can then begin the restoration process in the OXC 103.
This method suffers from several shortcomings. The communication of error information will have to occur through several elements of the system, most likely using software interfaces. These system components could potentially come from multiple vendors. In this arrangement, the likelihood of achieving the desired response times for network restoration is greatly decreased.
The present invention is directed to methods and apparatuses for detecting errors in the signals being transmitted through an optical switch and monitoring the position of the switch in order to improve network restoration time. The optical switch that the present invention operates on is composed of a micromachined mirror that is attached via a hinge to a substrate at an angle to the direction of the light beam so that when the mirror is parallel to and level with the substrate, in the xe2x80x9coffxe2x80x9d state, the light beam passes by it without disruption but when the mirror is perpendicular to the substrate, in the xe2x80x9conxe2x80x9d state, the light beam is redirected to another destination. These switches can be used in combination to form an optical cross connect for routing light beams between multiple destinations. A device of this type is fully described in a co-pending patent application entitled FIBER-OPTIC FREE-SPACE MICROMACHINED MATRIX SWITCHES, Ser. No. 09/001,676, filed Dec. 31, 1997 and is incorporated herein by reference.
The present invention monitors the light beam signal exiting the optical switch for errors and, if errors are detected, the invention narrows the possible causes by checking the error detecting sensor for failure and checking the position of the mirrors to ensure that they are set correctly for the proper transmission of the light beam.
The present invention monitors the signal for errors by using a beamsplitter placed in the path of the light beam to redirect a portion of the light beam onto a photodetector. The photodetector converts the light into an electronic signal that can be processed to detect loss of signal, loss of frame, and other errors using well known transmission error detection routines.
The present invention also provides three methods for detecting the state of the optical switch by monitoring the physical position of the micromachined mirrors.
The first method utilizes a circuit formed by conductive material along the mirror, the substrate, and along a probe mounted on the substrate that touches the conductive material on the mirror only when the mirror is substantially perpendicular to the substrate. The state of the mirror in the switch can be determined by the resistance of the circuit. If the circuit has finite resistance, the mirror is perpendicular and the switch is xe2x80x9con.xe2x80x9d If the circuit has a nearly infinite resistance, i.e., an open circuit, the mirror is not perpendicular and the switch is xe2x80x9coff.xe2x80x9d
The second method also utilizes a circuit formed by conductive material along the mirror, the substrate, and along a probe mounted on the substrate. However, the probe in this case does not touch the mirror when it is perpendicular to the substrate, but rather the probe is parallel to the mirror a short distance away. The position of the mirror can be determined by measuring the capacitance of the circuit created. The closer the mirror is to the perpendicular xe2x80x9conxe2x80x9d state, the higher the capacitance value will be.
The third method involves an additional optical input and output for each switch wherein a second light beam is generated by the optical input, reflected off the back of the mirror to the optical output, and the position of the mirror can be monitored based on the information returned by the light beam. If the light beam is reflected to the optical output, then the switch mirror is in the xe2x80x9conxe2x80x9d state; if the light beam is not reflected to the optical output, then the switch mirror is in the xe2x80x9coffxe2x80x9d state.